Return bend



Dec. 10, 1935. A. 'r. DUDLEY 2,023,805

RETURN BEND Filed Sept. 12, 1953 l 4 o l 14 23 I 2s "15 INVENTOR "15 2 ANDREW r DUDLEY 12 MAMA m1. M

ATTORN EY Patented Dec. 10, 1935 UNITED STATES RETURN BEND Andrew T. Dudley, Port Arthur, Tex., assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application September 12, 1933,-Serial No. 689,071

11 Claims.

This invention relates to a return bend assem-' bly employed in connection with pipe coils and the like and has particular reference to return bend connections for tubular heating systems in which fluids'are subjected to high temperatures and pressures'during their passage through the system, as for example, in the conversion of hy drocarbon oils wherein the oil is subjected to heat treatment as it passes at relatively high velocity and under relatively high pressure through a series of interconnected tubes.

In such a system a plurality of heating tubes are mounted in substantially parallel relation in a furnace setting, with the ends of the tubes ordinarily projecting through the walls of the furnace and the ends of adjacent tubes joined by return bend connections. a J

The severe temperature and pressure conditions to which these tubes are subjected in service make it desirable to connect the return bends with the tubes in such manner that they may be readily removed from the tubes so that the tubes" may be expeditiously removed, replaced, repaired or cleaned as required. This is particularly important when the fluid being heated is a decomposable substance, such as hydrocarbon oils, which tend to form a solid deposit on the interior of the tubes which may obstruct the tubes and create local hot spots over heating or buming, in which case the tubes must be cleaned or replaced.

Furthermore, successful operation requires that the removable return bend connections be capable of withstanding unequal movements of the various tubes, making up the integral parts of the heating coil, resulting from wide variations in temperature differences in different parts of the coil while at the same time maintaining a proper seal between the return bend connections and the tubes.

Another serious barrier to the construction of satisfactory return bond connections has been the inability to bring the ends of the tubes in proper alignment or registry for the application of the return bend connections. This is particularly true where the tubes extend in a vertical direction in which case it is necessary to support the bottom of the tubes while the top return bend connections are being installed and as a result it is extremely difiicult if not impossible to maintain the ends of the tubes in the same plane during assembly.

Heretofore it has been the practice to form the return bend connections of relatively rigid and inflexibleconstruction, the idea apparently being that rigidity was coupled with strength and that proper solution oi the problem resided in designing the return bend connections of the required rigidity to withstand the forces exerted without yielding or flexing. The result has been that the return bend connections are not only of' cumbersome and massive construction but are difiicult to install, requiring extreme accuracy in aligning the ends of the tubes and which in spite of strength and rigidity often developed serious 10 leaks requiring numerous replacements and repairs.

One of the primary objects of this invention is to overcome the difiiculties heretofore mentioned and to provide a return bend assembly which 15 does not depend upon rigidity, but rather permits a certain flexibility and relative movement between the several parts of the return bend assembly so thatstresses, resulting from misalignment of the tube ends when assembled, or relative movement of the tube ends under service conditions, are uniformly distributed to the different connecting parts and a uniform pressure is maintained about the periphery of the joint between the return bend fittings and the tube ends so that 25 35 Fig. 1 is aview in elevation showing parts in section of a return bend assembly constructed in accordance with the invention.

Fig. 2 is a sectional view on line 2-4 of Figure 1.

40 Fig. 3 is a sectional elevation view showing the assembly with the tubes out of alignment.

Referring now to the drawing, the return bend assembly in general comprises a return bend fit- ,ting ID in the form of U-shaped tubular conduit 45 adapted to abut against the ends of tubes II and provide a comiecting passage therebetween and removable clamping elements for securing the return bend I!) to the tubes H and efiect a fluid tight seal therebetween. While the drawing illus- 50 trates the invention as applied to two adjacent tubes it is to be understood that the heating coil or tubular heating system is ordinarily made up of a considerable number of such tubes extending through a furnace setting and having the adjoining ends connected in a similar manner.

The invention finds particular advantage in furnaces in which the tubes are arranged in vertical direction, the several tubes being supported at their upper ends in such manner that the expansion resulting from the furnace temperature is reflected only in a downward direction. beforepointed out in this type of furnace it is extremely difficult to bring the ends of the tubes into accurate alignment. However, the invention is also applicable to other arrangements in which the tubes are assembled in other positions such as horizontal tubes extending longitudinally or transversely of the furnace. It will be also clear that the tubes need not necessarily be mounted in a furnace but may be mounted in any desired apparatus such as heat exchangers or economizers in which the tubes are subjected to relatively high temperatures. While the return bend fitting II] has been illustrated and described as being substantially U-shaped, it is evident that the invention will be equally applicable to fittings of other configurations such as elbow (L-shaped) or other angular coupling members.

The ends of the tubes II are expanded to form shoulders I2 adapted to engage complementary shoulders I3 of terminal members I4 as illustrated in Figures 1 and 3. The terminal members I4, which form a part .of the clamping elements heretofore referred to, are roughly in the shape of a U as shown in Figure 2 with a tubular base I5 adapted to fit over the ends of tubes II and having a configuration corresponding to the outer surface of the tube ends. One convenient method of securing the terminal members I4 to the ends of tubes I I is to slide the tubular base I5 over the ends of the tube so that the ends project outwardly from the base, and then expand the walls of the tube by a swaging or rolling operation, into snug engagement against the shoulder I3 of the tubular base.

While this method of securing the terminal members I4 to the tubes II is deemed preferable and has'been found to be effective and economical, it will be manifest that the parts may be secured by other methods well known to those skilled in this particular art. By way of example the terminal members I4 may be threaded to upset ends of the tubes I I or the ends of the tubes may be provided with an outwardly circumferential bead to which the tubular base may be either secured directly or through the medium of a split ring fitted between the tubular base and the tubes. The upper edge portions of the tubular bases I5 are provided with aligned slots I6 into which a web ll of the return bend I0 extends.

The upwardly projecting arms I8 of the U- shaped terminal members vI4 are assembled to straddle opposite ends of the return bend fitting I0 and the arms I8 of the individual terminal members I4 are connected intermediate their lengths by a bar I9 to prevent the legs from spreading when pressure is applied and to form a stop against which lugs 20 on the return bend fitting III- is adapted to abut to limit the movement of the return bend member between theterminal members I4.

The abutting edges 2| and 22 of the tubes II and the return bend fitting II] respectively, are ground smooth and curved, the center of curvature being preferably located on the axis of the tube so as to form a ground valve connection of the ball and socket type which enables the return bend fitting I 0 to tilt longitudinally to a. limited extent while maintaining a requisite bearing surface at the joint as illustrated in Figure 3. The mouths 23 of the return bend fitting I0 immediately adjacent the ball and socket joint are of slightly enlarged diameter so that limited tilting of the return bend fitting to adjust itself to misalignment of the tube ends will not materially affect the flow of fluid through the fitting.

The upper ends of the legs I8 are provided with inwardly projecting ledges 24, best illustrated in Figure 2, having the bottom edges curved andundercut to provide downwardly and outwardly inclined faces 25 against which complementary faces 26 on the opposite edges of screw caps 21 are adapted to bear. As best illustrated in Figures 1 and 3 the curvature of the inclined faces 25 of the terminal members I4 and the complementary bearing faces 28 on the screw caps 21 are concentric to the curvature of the end edges 2I and 22 forming the ball and socket joint.

The inclined faces of the bearing surfaces 25 of the terminal members and the complementary bearing faces 26 on the screw caps 21 tend to prevent lateral spreading of the arms I8 when pressure is applied thereto.

The screw caps 21 are provided with threaded openings through which clamping screws or bolts 28 extend, the lower ends of which are preferably crowned and adapted to engage in depressions 29 on the top of the return bend Ill. The upper ends of screws or bolts 28 are provided with hexagonal heads or other suitable means whereby rotation may be imparted to the bolts or screws for applying pressure between the return bend I0 and tubes II to effect the desired seal therebetween.

In assembling the return bend unit designed according to the invention the tubular bases I5 of terminal members I4 are slipped over the ends of the tubes so that the ends of the tubes project sufficiently beyond the tubular base to afford the desired clearance for the ball and socket joint and the ends of the tubes I I are then swaged or rolled into tight engagement with the inner walls of the tubular bases I5. The return bend fitting I0 is then placed in position with the web I I extending into aligned slots I6 on the terminal members and the ends of the return bend fitting in proper registry with ends of the tubes I I, it being understood that the opposing edges have previously been machined in the manner hereinbefore described. The screw caps 21 are then placed in po sition between the terminal arms I8 and the stud bolts 28 tightened against the return bend fitting I0 until sufiicient pressure is applied to effectively seal the ball and socket joints.

As illustrated in Figure 3 it is not essential that the ends of the tubes be accurately aligned as the ball and socket joint may be designed to permit the return bend fitting ID to adapt itself to any normal misalignment of the tube ends by properly proportioning the width of the bearing surfaces of the ball and socket joint to provide the necessary bearing area around the periphery of the joint to effect a fluid tight seal thereabout, and by proportioning the diameter of the mouths 23 of the return bend fitting I0 so as not to obstruct or affect the flow of fluid through the fitting. As illustrated in Figure 3 the mouths 23 of the fitting III are preferably enlarged to a diameter such that the diameter of the fluid passage at the joint will not be less than the di'- ameter of the tubes when the fitting is tilted to its extreme position. It will be self evident that the same result may be obtained by enlarging the mouths of the tube rather than the fitting.

As an example, in tubular heating systems for the conversion of hydrocarbon oils, it has been found that return bend connections for five inch diameter tubes designed to adjust themselves to variations up to an inch in the elevation of the tube ends will prove satisfactory under the usual assembly and operating conditions. The design of the fitting with respect to the amount of adjustment for misalignment will be determined largely by the conditions to which it is subjected and greater or less adjustment may be made as occasion requires.

In order to insure that a proper bearing area about the periphery of the ball and socket joint is maintained to provide the required seal, the return bend fitting I is provided with lugs 20 which are adapted to abut against the bars l9 and prevent further rocking or tilting movement after the fitting has been tilted a predetermined amount as before pointed out.

By permitting the screw caps 21 to slide in a curved path concentric to the curvature of the ball and socket joint the screw caps may adjust themselves to any tilted position of the return bend fitting to so that the application of pressure to the ends of the fitting, necessary to effect a fluid tight seal at the joints, will be in a direction directly opposed to the direction of thrust on the ends of the bolts, thus insuring uniform pressure about the periphery of the joint and preventing localized concentrated stresses in different parts of the assembly.

The work of assembling the tubular heater is ordinarily done when the various parts are cold. During operation heat is applied to the tubes,

causing the tubes to expand, the amount of expansion depending upon the coefiicient of expansion of the tubes and the temperature to which they are heated. As a result there is an unequal expansion of the tubes in the hotter and cooler portions of the heating zone tending to cause mis alignment of the tube ends.

In additiorfit is sometimes desirable to use special alloy tubes such as a chromium, nickel, iron alloy in the hotter portions of the heating zone, or in the fittings, in order to reduce the corrosive action on the tubes or for obtaining the desired strength, in which case the dififerent coefiicient of expansion of the'alloytubes or fittings may accentuate the relative movement and misalignment of the tube ends. Furthermore, in many cases the ends of the tubes project through the furnace walls and the return bend fittings are mounted on the outside of the furnace so that there is a material difference in temperature between the return bend fittings and tubes thus normally tending to aggravate the difficulty in maintaining accurate alignment. Likewise the return bend fittings and tubes will expand more rapidly and to a greater extent per unit length than the clamping elements. Another factor adding to the difficulty of maintaining accurate alignment of the tube ends is the tendency of the. tubes to sag as a result of the high temperature in the heating zone.

These factors together with others tending to cause misalignment of the tube ends under operating conditions also result in setting up stresses in the joints which, if rigid, become highly concentrated and localized and often result in failure of the part under concentrated stress, or the pressure on a portion of the periphery of the joint may be relieved to an extent which will cause leakage.

By making the return bend unit flexible in the manner heretofore described so that the different parts of the unit may move relative to each other 5 to accommodate themselves to different positions of the tube ends, the stresses will be uniformly distributed over the bearing surface, and a uniform pressure may be maintained about the periphery of the joint as hereinbefore pointed out.

While the preferred embodiment has been de scribed for purposes of illustration it is understood that the invention embraces such other modifications or variations as come within the spirit and scope thereof and that the invention is not to be limited except as necessary to distinguish from prior art.

I claim:

1. A return bend assembly for connecting the ends of a pair of tubular members to provide a fluid passage therebetween comprising a fitting having a return passage therethrough adapted to register with the ends of said tubular members, pivotable joints between said fitting and tubular members to permit tilting of said fitting to adjust the same to misalignment of the ends of said tubular members, bridge members spanning said fitting adjacent the ends thereof, elongated pressure applying members movably connected to said bridge members adapted to engage the end portions of said fitting opposite said tubular members, means for resisting backward pressure from said pressure applying means so constructed and arranged as to directly oppose thrust thereon when said fitting is in different tilted positions.

2. A return bend assembly for connecting tubular members to provide a fluid passage therebetween, comprising a fitting having an arcuate passage adapted to align with the ends of the tubular members, ball and socket joints between said fitting and the ends of said tubular members to provide for tilting of said fitting to adjust the same to misalignment of the ends of the tubular members, and means for applyingpressure between said fitting and the ends of said tubular members to effect a fluid tight seal therebetween, said last named means including blocks bridging the fitting adjacent the ends thereof, means engaging opposite sides of said block, said blocks being movably connected to said engaging means and correlated to the curvature of the ball and socket joint to equalize stresses when said fitting is in tilted position.

3. A return bend assembly for connecting tubular members to provide a fluid passage therebetween, comprising a fitting having an arcuate passage adapted to align with the ends of said tubular members, rockable joints between said fitting and said tubular members to permit tilting of said fitting, terminal members connected to said tubular members having portions thereof projecting on opposite sides of said fitting, backing blocks bridging said fitting adjacent the ends thereof, rockable connections between said blocks and the projecting portions of said terminal members correlated to said rockable joints to equalize stresses when said fitting is in tilted position, and pressure applying means connecting said backing blocks and said fitting for applying and maintaining pressure between said fitting and the ends of said tubes to eflect a fluid seal therebetween.

4. In a tubular heating system in which a fluid is heated during its passage through a plurality of interconnected tubes, a return bend assembly for connecting ends of adjacent tubes, said assembly comprising a. fitting having a return passage, the opposite ends of said passage being adapted to align with the ends of adjacent tubes, curved bearing faces on the end of said fitting having the center of curvature located on the axis of said tubes and adapted to engage complementary curved bearing faces on the ends of said tubes, terminal members secured to the ends of each of said tubes and having portions thereof straddling the ends of said fitting, curved bearing faces on said terminal members disposed on opposite sides of said fitting concentric to the curvature of the bearing faces at the ends of said fitting, movable blocks disposed between said bearing faces, complementary curved bearing faces on said blocks adapted to slidably engage the bearing faces of said terminal members, and means disposed between said block and fitting for applying pressure between said tubes and fitting to effect a fluid tight joint between said fitting and the ends of said tubes.

5. A return bend assembly for connecting ends of adjacent tubular members, said assemblycomprising a fitting having an arcuate passage adapted to align with the ends of said tubular members and provide a fluid passage therebetween, bearing faces at the ends of said fitting adapted to engage complementary bearing faces at the ends of said tubular members, terminal members connected to each of said tubular members having portions thereof straddling said fitting, blocks movable in an arcuate pathway carried by said terminal members and bridging said fitting adjacent the ends thereof, and movable pressure applying means associated with said block for applying pressure between said fitting and tubular members to efl'ect a fluid tight joint therebetween.

6. A return bend assembly for connecting the ends of tubular members and provide a fluid passage therebetween comprising'a fitting having an arcuate passage adapted to align with the ends of said tubular members, ball and socket joints between said fitting and said tubes permitting tilting of said fitting relative to said tubes, enlarged outlets at the ends of said passage, terminal members connected to the ends of said tubular members having portions thereof extending on opposite sides of the fitting, backing blocks bridging said fitting adjacent the ends thereof and oscillatably mounted in said terminal members to equalize stresses when said fitting is in tilted position, and pressure applying means connecting said fitting and blocks for applying pressure between said fitting and the ends of said tubular members to provide a fluid seal therebetween.

7. A return bend assembly for connecting tubular members to provide a fluid passage therebetween comprising a fitting having an arcuate passage adapted to align with the ends of said tubular members, rockable joints between said fitting and said tubular members to permit tilting of said fitting to adjust the same to misalignment of said tubular members, means limiting the tilt of said fitting, terminal members connected to said tubular members having portions thereof projecting on opposite sides of said fitting, backing blocks bridging said fitting adjacent the ends thereof, rockable connections between said blocks and the projecting portions of said terminal members correlated to said rockable joint to equalize stresses when said fitting is in tilted position, pressure applying means connecting said backing block and said fitting for applying and maintaining pressure between said fitting and the ends of said tubular members to effect a fluid seal therebetween.

8. A return bend assembly for connecting tubular members to provide a fluid passage therebe- 5 tween comprising a fitting having a return passage adapted to align with the ends of said tubular members, a ball and socket joint between said fittings and the ends of said tubular members to permit limited tilting of said fitting to adjust l0 the'same to misalignment of the ends of the tubular members, tubular sleeves encompassing the ends of said tubular members, expanded portions at the ends of said tubular members engaging complementary surfaces within said tubular sleeve to prevent displacement thereof, integral arms on said sleeves projecting on opposite sides of the ends of said fittings, screw blocks disposed between said arms and bridging said fitting adjacent the ends thereof, oscillatable connections between said screw blocks and said arms to equalize stresses when said fitting is in tilted position, bolts threadably connected to said blocks and having end bearing faces adapted to engage the ends of said fitting, and means whereby said bolts may be rotated to apply pressure between said fitting and the ends of said tubes to effect a fluid tight seal therebetween.

9. A return bend assembly for connecting tubular members to provide a fluid passage therebetween comprising a fitting having a return passage adapted to align with the ends of said tubular members, rockable joints between said fitting and said tubular members to permit tilting of said fitting to adjust the same to misalignment of said tubular members, means limiting the tilt of said fitting, enlarged outlets for said return passage, terminal members secured to said tubular members having portions thereof projecting on opposite sides of said fitting, screw blocks bridging said fitting adjacent the ends thereof, rockable connections between said screw blocks and the projecting portions of said terminal members correlated with said rockable joints to equalize stresses when said fitting is in tilted position, bolts threadably connected to said screw blocks and adapted to engage the ends of said fitting, means whereby said bolts may be rotated for applying pressure between said fitting and said tubes to effect a fluid tight seal therebetween.

10. In a heat exchange apparatus for the heat treatment of hydrocarbon oils having a plurality of tubes extending in substantially parallel relation, return bends communicating with the ends of adjacent pairs of tubes, pivotable joints between the ends of said return bends and the ends of said tubes to permit movement of said return bends to adjust themselves to misalignment of said tubes, terminal members connected to the ends of said tubes and having portions thereof extending on opposite sides of said return bends, blocks bridging said return bends adjacent the ends thereof, movable connections between said blocks and terminal members correlated with said pivotable joints to equalize stresses when said ()5 tubes are in misalignment and means mounted in said blocks for applying sealing pressure be tween said return bends and the ends of said tubes.

11. In a tubular heating system for the heat treatment of hydrocarbon oils having a plurality of tubes arranged in vertical direction and having pairs of tubes extending in substantially parallel relation, return bend fittings communieating with the ends of adjacent pairs of tubes,

ball and socket joints between said fitting and the ends of said tubes to permit tilting of said fittings to adjust themselves to misalignment of said tubes, terminal members connected to said tubes having portions thereof disposed on opposite sides of said fitting, bearing blocks bridging said fittings adjacent the ends thereof, complementary curved bearing surfaces between said bearing blocks and terminal members having curvatures concentric to the curvature of the hearing faces of the ball and socket joint to equalize stresses when said fitting is in tilted position and means mounted in said blocks for applying sealing pressure between said fittings and the ends of said tubes.

ANDREW T. DUDLEY. 

